Integrated process for producing ethanol

ABSTRACT

Processes and systems for forming ethanol from methanol. The process involves carbonylating the methanol to form acetic acid and hydrogenating the acetic acid to form ethanol. In a first aspect, at least some hydrogen for the hydrogenating step is derived from a tail gas stream formed in the carbonylation step. In a second aspect, at least some carbon monoxide for the carbonylation step is derived from a vapor stream in the hydrogenation system. In a third aspect, a syngas stream is separated to form a hydrogen stream and a carbon monoxide stream, and the hydrogen stream is methanated to remove residual carbon monoxide prior to being introduced into the hydrogenation system.

FIELD OF THE INVENTION

The present invention relates generally to processes for producingethanol and, in particular, to producing ethanol from acetic acid.

BACKGROUND OF THE INVENTION

Ethanol for industrial use is conventionally produced from petrochemicalfeed stocks, such as oil, natural gas, or coal, from feed stockintermediates, such as syngas, or from starchy materials or cellulosematerials, such as corn or sugar cane. Conventional methods forproducing ethanol from petrochemical feed stocks, as well as fromcellulose materials, include the acid-catalyzed hydration of ethylene,methanol homologation, direct alcohol synthesis, and Fischer-Tropschsynthesis. Instability in petrochemical feed stock prices contributes tofluctuations in the cost of conventionally produced ethanol, making theneed for alternative sources of ethanol production all the greater whenfeed stock prices rise. Starchy materials, as well as cellulosematerial, are converted to ethanol by fermentation. However,fermentation is typically used for consumer production of ethanol, whichis suitable for fuels or human consumption. In addition, fermentation ofstarchy or cellulose materials competes with food sources and placesrestraints on the amount of ethanol that can be produced for industrialuse.

Ethanol production via the reduction of alkanoic acids and/or othercarbonyl group-containing compounds has been widely studied, and avariety of combinations of catalysts, supports, and operating conditionshave been mentioned in the literature. In addition, integrated processesfor making ethanol from various raw materials, such as biomass, viasyngas, methanol and acetic acid intermediates, have also been proposed.See, e.g., U.S. Pat. Nos. 7,608,744; 7,863,489 and 7,884,253, theentireties of which are incorporated herein by reference. The needremains for improved processes for producing ethanol from commerciallyavailable materials, and in particular, for processes for formingethanol having improved overall conversion and selectivity.

SUMMARY OF THE INVENTION

The present invention relates to processes for forming ethanol fromacetic acid, which preferably is formed from the carbonylation ofmethanol. In one embodiment, the present invention is directed to aprocess for forming an alcohol, comprising: (a) separating, e.g.,membrane separating, a syngas stream comprising carbon monoxide andhydrogen into a hydrogen stream comprising hydrogen and residual carbonmonoxide, and a carbon monoxide stream comprising carbon monoxide; (b)methanating at least a portion of the hydrogen stream to convert atleast a portion of the residual carbon monoxide to methane and water andforming a methanated hydrogen stream; and (c) hydrogenating an alkanoicacid with hydrogen from the methanated hydrogen stream in the presenceof a catalyst, optionally comprising platinum or palladium, underconditions effective to form the alcohol.

In another embodiment, the invention is to a process for forming analcohol from an alkanoic acid, comprising separating (optionally with amembrane) and methanating syngas to form a methanated hydrogen streamand hydrogenating the alkanoic acid to form the alcohol, wherein atleast some hydrogen for the hydrogenating step is derived from themethanated hydrogen stream. The separating preferably comprisesseparating the syngas into a hydrogen stream and a carbon monoxidestream.

In either embodiment, the alkanoic acid preferably is acetic acid andthe alcohol preferably is ethanol. Optionally, the process may be anintegrated process further comprising carbonylating an alcohol,preferably methanol, having n carbon atoms with carbon monoxide from thecarbon monoxide stream to form the alkanoic acid, wherein the alkanoicacid, preferably acetic acid, and the alcohol formed in thehydrogenating step, preferably ethanol, each comprise n+1 carbon atoms.

The methanated stream preferably comprises at least 85 vol. % hydrogenand preferably less than 2 vol. % carbon monoxide. The methanolpreferably is derived from a carbon source selected from the groupconsisting of natural gas, oil, petroleum, coal, biomass, andcombinations thereof.

In another embodiment, the invention is to a system for forming analcohol, comprising: (a) a separation unit, e.g., a membrane separationunit, for separating a syngas stream comprising carbon monoxide andhydrogen into a hydrogen stream comprising hydrogen and residual carbonmonoxide, and a carbon monoxide stream comprising carbon monoxide; (b) amethanation unit in fluid communication with the separation unit formethanating at least a portion of the hydrogen stream to convert atleast a portion of the residual carbon monoxide to methane and water andforming a methanated hydrogen stream; and (c) a hydrogenation reactor influid communication with the methanation unit for hydrogenating analkanoic acid with hydrogen from the methanated hydrogen stream in thepresence of a catalyst under conditions effective to form the alcohol.The system optionally further comprises a carbonylation unit in fluidcommunication with the separation unit for carbonylating an alcoholhaving n carbon atoms with carbon monoxide from the carbon monoxidestream to form the alkanoic acid, wherein the alkanoic acid and thealcohol formed in the hydrogenating step comprise n+1 carbon atoms, thesystem further comprising a conduit in fluid communication between thecarbonylation unit and the hydrogenation unit for directing the alkanoicacid from the carbonylation unit to the hydrogenation unit.

BRIEF DESCRIPTION OF DRAWINGS

The invention is described in detail below with reference to theappended drawings, wherein like numerals designate similar parts.

FIG. 1 is a schematic flow diagram of a first embodiment of theinvention.

FIG. 2 is a schematic flow diagram of a second embodiment of theinvention.

FIG. 3 is a schematic flow diagram of a third embodiment of theinvention.

FIG. 4 is a flow chart of acetic acid high-pressure tail gas process,which includes a gas pre-treatment system before feeding to a membraneseparator and which may be adopted in the first embodiment of theinvention.

FIG. 5 is a schematic flow chart of acetic acid high-pressure tail gasrecovery for the first embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Introduction

In a first embodiment, the present invention relates to processes forproducing ethanol from methanol via an acetic acid intermediate. Theprocess involves the carbonylation of methanol in a carbonylationreactor to form acetic acid and hydrogenating the acetic acid in ahydrogenation reactor with hydrogen derived, at least in part, from atail gas stream formed in the carbonylation process. Carbon monoxide(CO) may also be recovered from the tail gas and recycled to thecarbonylation step.

In a second embodiment, the crude ethanol product is sent from thehydrogenation reactor to a separator unit, e.g., flash vessel, andcarbon monoxide is recovered from separator unit and is recycled to thecarbonylation reactor to form additional acetic acid. Hydrogen may alsobe recovered from the separator unit and recycled to the hydrogenationstep and/or recycled to enhance stabilization of the carbonylationcatalyst.

In a third embodiment, a syngas stream comprising carbon monoxide andhydrogen is separated into a hydrogen stream comprising hydrogen andresidual carbon monoxide, and a carbon monoxide stream comprising carbonmonoxide. At least a portion of the hydrogen stream is methanated toconvert at least a portion of the residual carbon monoxide to methaneand water and forming a methanated hydrogen stream. An alkanoic acid,preferably acetic acid, is hydrogenated with hydrogen from themethanated hydrogen stream in the presence of a catalyst underconditions effective to form the alcohol, preferably ethanol.Optionally, all or a portion of the carbon monoxide stream may bedirected to a carbonylation unit for the synthesis of the alkanoic acid,preferably acetic acid.

FIGS. 1-3 illustrate process flow schemes according to the threeembodiments of the invention. As shown, carbonylation reactor 1 receivesa carbon monoxide stream 2 and a methanol stream 3 and forms a crudeacetic acid stream 4. Crude acetic acid stream 4 is separated in aseparation zone 5 to form a purified acetic acid stream 6, which is sentto a hydrogenation reactor 7. In hydrogenation reactor 7, acetic acidfrom purified acetic acid stream 6 is hydrogenated with hydrogen fromhydrogen stream 8 to form a crude ethanol product 9, which is sent to aseparation zone 10 to form a final ethanol product 11.

According to a first embodiment of the invention, shown in FIG. 1, tailgas 12 comprising carbon monoxide and hydrogen is sent to a membraneseparator 13, optionally after pretreating as described below withreference to FIGS. 4 and 5. Membrane separator 13 preferably separatesthe tail gas into a carbon monoxide product stream 14 and a hydrogenstream 15. As discussed herein, membrane separator 13 may comprise asingle membrane separation unit or multiple membrane separation units toprovide a hydrogen stream 15 having the desired hydrogen concentration.Suitable membranes include shell and tube membrane modules having one ormore porous material elements therein. Non-porous material elements mayalso be included. The material elements may include polymeric elementsuch as polyvinyl alcohol, cellulose esters, and perfluoropolymers.Membranes that may be employed in embodiments of the present inventioninclude those described in Baker, et al., “Membrane separation systems:recent developments and future directions,” (1991) pages 151-169, Perryet al., “Perry's Chemical Engineer's Handbook,” 7th ed. (1997), pages22-37 to 22-69, the entireties of which are incorporated herein byreference. Hydrogen permeable membranes are preferred.

The carbon monoxide product stream 14 is preferably recycled to thecarbonylation reactor 1, and hydrogen stream 15 preferably is sent tomethanation unit 16. Residual carbon monoxide contained in the hydrogenstream 15 is converted to methane in methanation unit 16 to form amethanated hydrogen stream 17, which is sent to hydrogenation reactor 7.Fresh hydrogen via stream 18 optionally is combined with methanatedhydrogen stream 17 to form hydrogen stream 8, which is sent to thehydrogenation reactor 7.

In another embodiment, not shown, methanation unit 16 is replaced with acarbonylation reactor, optionally a secondary carbonylation reactor,which operates to reduce the carbon monoxide concentration by reactingthe carbon monoxide with methanol, which is optionally added to stream15 or to the carbonylation reactor, to form acetic acid. The acetic acidmay then be added to the hydrogenation reactor 7 to form additionalethanol. See, e.g., U.S. patent application Ser. No. 12/892,348, theentirety of which is incorporated herein by reference, which describesthe use of a secondary carbonylation reactor.

In the second embodiment of the invention, shown in FIG. 2, crudeethanol product 9 is sent to a separator 19, e.g., a flash vessel orabsorber (optionally a methanol absorber to remove residual aceticacid), which forms flash stream 20 and a liquid crude products stream21. Flash stream 20 comprises hydrogen and a minor amount of carbonmonoxide, and is directed to a membrane separator 26, optionally afterpretreating as described below. Membrane separator 26 preferablyseparates carbon monoxide in carbon monoxide product stream 22 from ahydrogen stream 23. The carbon monoxide product stream 22 is preferablyrecycled to the carbonylation reactor 1, and hydrogen stream 23preferably is sent to methanation unit 24. Residual carbon monoxidecontained in the hydrogen stream 23 is converted to methane inmethanation unit 24 to form a methanated hydrogen stream 25, which issent to hydrogenation reactor 7. Fresh hydrogen via stream 27 optionallyis combined with methanated hydrogen stream 25 and sent to thehydrogenation reactor 7.

In another, not shown, methanation unit 24 is replaced with acarbonylation reactor, which operates to reduce the carbon monoxideconcentration by reacting the carbon monoxide with methanol, which isoptionally added to stream 23 or to the carbonylation reactor, to formacetic acid. The acetic acid may then be added to the hydrogenationreactor 7 to form additional ethanol.

In the third embodiment of the invention, a syngas stream 29 comprisinghydrogen and carbon monoxide is sent to a membrane separator 30,optionally after pretreating as described below with reference to FIGS.4 and 5. Membrane separator 30 preferably separates carbon monoxide incarbon monoxide product stream 28 from a hydrogen stream 31. The carbonmonoxide product stream 28 is optionally directed to carbonylationreactor 1 to serve as the carbon monoxide source for the carbonylationof methanol (or other alcohol), and hydrogen stream 31 preferably issent to methanation unit 32. Residual carbon monoxide contained in thehydrogen stream 31 is converted to methane in methanation unit 32 toform a methanated hydrogen stream 33, which is sent to hydrogenationreactor 7. Fresh hydrogen via stream 34 optionally is combined withmethanated hydrogen stream 33 to form a combined hydrogen stream that issent to the hydrogenation reactor 7.

In another embodiment, the various embodiments may be combined togetherin various combinations. For example, the first embodiment of theinvention may be combined with the second embodiment of the invention,the second embodiment may be combined with the third embodiment, thefirst embodiment may be combined with the third embodiment, or all threeembodiments may be combined. Although the subject disclosure is directedprimarily to the use of membrane separation units for separatinghydrogen/carbon monoxide streams into a hydrogen stream and a carbonmonoxide stream, other separation techniques may also be used, e.g.,liquefaction or by separation columns, e.g., cryogenic separationcolumns, pressure swing adsorption (PSA) units, amine scrubbing, and thelike.

Acetic Acid Synthesis

The present invention relates to processes for hydrogenating acetic acidto form ethanol. The acetic acid, in turn, is formed from thecarbonylation of methanol. Methanol carbonylation processes suitable forproduction of acetic acid are described in U.S. Pat. Nos. 7,208,624;7,115,772; 7,005,541; 6,657,078; 6,627,770; 6,143,930; 5,599,976;5,144,068; 5,026,908; 5,001,259; and 4,994,608, the entire disclosuresof which are incorporated herein by reference.

As petroleum and natural gas prices fluctuate becoming either more orless expensive, methods for producing acetic acid and intermediates suchas methanol and carbon monoxide from alternate carbon sources have drawnincreasing interest. In particular, when petroleum is relativelyexpensive, it may become advantageous to produce acetic acid fromsynthesis gas (“syngas”) that is derived from more available carbonsources. U.S. Pat. No. 6,232,352, the entirety of which is incorporatedherein by reference, for example, teaches a method of retrofitting amethanol plant for the manufacture of acetic acid. By retrofitting amethanol plant, the large capital costs associated with carbon monoxidegeneration for a new acetic acid plant are significantly reduced orlargely eliminated. All or part of the syngas is diverted from themethanol synthesis loop and supplied to a separator unit to recovercarbon monoxide, which is then used to produce acetic acid. In a similarmanner, hydrogen for the hydrogenation step may be supplied from syngas,although in the first embodiment at least a portion of the hydrogen inthe hydrogenation step preferably is derived from the tail gas formed inthe carbonylation process, and in the second embodiment at least aportion of the hydrogen in the hydrogenation step is recycled from aseparator vessel downstream of the hydrogenation reactor.

In some embodiments, some or all of the raw materials for the aceticacid hydrogenation process may be derived partially or entirely fromsyngas. For example, the acetic acid may be formed from methanol andcarbon monoxide, both of which may be derived from syngas. The syngasmay be formed by partial oxidation reforming or steam reforming, and thecarbon monoxide may be separated from syngas. Similarly, hydrogen thatis used in the step of hydrogenating the acetic acid to form the crudeethanol product may be separated from syngas, although in the firstembodiment at least a portion of the hydrogen preferably is derived fromthe tail gas formed in the carbonylation process. The syngas, in turn,may be derived from variety of carbon sources. The carbon source, forexample, may be selected from the group consisting of natural gas, oil,petroleum, coal, biomass, and combinations thereof. Syngas or hydrogenmay also be obtained from bio-derived methane gas, such as bio-derivedmethane gas produced by landfills or agricultural waste.

Examples of biomass include, but are not limited to, agriculturalwastes, forest products, grasses, and other cellulosic material, timberharvesting residues, softwood chips, hardwood chips, tree branches, treestumps, leaves, bark, sawdust, off-spec paper pulp, corn, corn stover,wheat straw, rice straw, sugarcane bagasse, switchgrass, miscanthus,animal manure, municipal garbage, municipal sewage, commercial waste,grape pumice, almond shells, pecan shells, coconut shells, coffeegrounds, grass pellets, hay pellets, wood pellets, cardboard, paper,plastic, and cloth. See, e.g., U.S. Pat. No. 7,884,253, the entirety ofwhich is incorporated herein by reference. Another biomass source isblack liquor, a thick, dark liquid that is a byproduct of the Kraftprocess for transforming wood into pulp, which is then dried to makepaper. Black liquor is an aqueous solution of lignin residues,hemicellulose, and inorganic chemicals.

U.S. Pat. No. RE 35,377, also incorporated herein by reference, providesa method for the production of methanol by conversion of carbonaceousmaterials such as oil, coal, natural gas and biomass materials. Theprocess includes hydrogasification of solid and/or liquid carbonaceousmaterials to obtain a process gas which is steam pyrolized withadditional natural gas to form synthesis gas. The syngas is converted tomethanol which may be carbonylated to acetic acid. The method likewiseproduces hydrogen which may be used in connection with this invention asnoted above. U.S. Pat. No. 5,821,111, which discloses a process forconverting waste biomass through gasification into synthesis gas, andU.S. Pat. No. 6,685,754, which discloses a method for the production ofa hydrogen-containing gas composition, such as a synthesis gas includinghydrogen and carbon monoxide, are incorporated herein by reference intheir entireties.

In the first embodiment, a process and unit for carbon monoxide andhydrogen recovery from high-pressure tail gas of carbonylation to aceticacid by membrane method are provided, in which the tail gas is subjectedto pre-treatment, membrane separation, and gas compression. Aspects ofthis process may also be adapted to the second embodiment of theinvention. The tail gas passing through a gas washing column, a misteliminator, and a combined filter is heated by a heater, and thencharged into a membrane separator, e.g., a Prism membrane separator; therecovered carbon monoxide product gas after membrane separation ischarged into a compressor, the carbon monoxide is recovered for reuse,and (for the first embodiment) the hydrogen-rich gas (hydrogen stream)is charged to a hydrogenation reactor, optionally after a methanationstep, for the production of ethanol, as discussed below. By using twosets of membrane systems, e.g., Prism membrane separator systems, thegas amounts of the raw gas fed into the membrane, the product gas, andthe hydrogen-rich gas are controlled to reach a N₂ material balance, toensure carbon monoxide and hydrogen recovery rate, so as to achievelong-term stable running of the membrane separation unit. The content ofcarbon monoxide in the final product gas is preferably equal to orgreater than 91 vol. %. Processes for separating carbon monoxide from anacetic acid tail gas stream are described in CN 101439256 A, theentirety of which is incorporated herein by reference.

The present invention provides a process for carbon monoxide andhydrogen recovery by one or more membranes from high-pressure tail gasof a methanol carbonylation reactor, in which carbon monoxide andhydrogen in the tail gas after pre-treatment are recovered by a membraneseparation method, and the carbon monoxide content in the product gas isequal to or greater than 91% (molar percentage). The resulting hydrogenstream (before optional methanation) preferably comprises at least 18vol. % hydrogen, e.g., at least 20 vol. % hydrogen, at least 25 vol. %hydrogen, at least 50 vol. % hydrogen, at least 75 vol. % hydrogen or atleast 85 vol. % hydrogen. In terms of ranges, the resulting hydrogenstream optionally comprises from 18 to 99 vol. % hydrogen, from 25 to 97vol. % hydrogen, or from 75 to 95 vol. % hydrogen. The hydrogen contentof the hydrogen enriched stream may be advantageously increased to adesired level by using multiple separation stages, if desired.

In one embodiment, the technical solution of the present invention is aprocess for carbon monoxide and hydrogen recovery from high-pressuretail gas of carbonylation to acetic acid by a membrane method, in whichthe tail gas is subjected to three steps: pre-treatment, membraneseparation, and gas compression. The resulting hydrogen stream isdirected to a hydrogenation reactor for converting acetic acid toethanol, as described below.

In one optional pretreatment process, a high-pressure mixed acetic acidtail gas at a temperature of 40-60° C., a pressure of 2.6-2.8 MPa, and aflow rate of 2000 m³/h-6000 m³/h is charged into a gas washing column,and contacts top-down desalted water reversely in the column. The aceticacid content in the acetic acid tail gas at the outlet of the gaswashing column may be equal to or less than 100 wppm, and thecirculation of the desalted water of the gas washing column may be from0.5-1.0 m³/h. The tail gas from the gas washing column may be chargedinto a mist eliminator to further remove droplets entrained therein, andthe liquid level of the mist eliminator may be controlled at 150-200 mm.The tail gas from the mist eliminator may be charged into a combinedfilter with a filter accuracy of about 0.01 μm. The tail gas from thecombined filter may be heated to 40-60° C. with a heater.

In an optional membrane separation step, the tail gas afterpre-treatment is charged into a membrane separator, e.g., a Prismmembrane separator, at a pressure increasing rate equal to or less than0.3 MPa/min.

In an optional gas compression step, the tail gas, specifically theseparated carbon monoxide product gas derived therefrom, after membraneseparation is charged into a compressor at a flow rate of 1000 m³/h-3000m³/h. The carbon monoxide product gas is preferably recovered for reuse,and the hydrogen-rich gas (hydrogen stream) is preferably charged to ahydrogenation unit with acetic acid to form ethanol.

A unit used in the process for carbon monoxide and hydrogen recoveryfrom high-pressure tail gas of carbonylation to acetic acid by membranemethod mainly includes a gas washing column, a mist eliminator, acombined filter, a heater, a membrane separator, and a compressor. Thegas washing column is connected to the mist eliminator through aconduit, the mist eliminator is connected to the combined filter througha conduit, the combined filter is connected to the heater through aconduit, the heater is connected to the separator through a conduit, andthe separator is connected to the compressor through a conduit. Themembrane separator optionally comprises a total of 20 to 50 membranecomponents, preferably about 32 membrane components in one, two or moresets, although in some embodiments, more membrane components and stagesmay be desired to form a hydrogen stream of sufficiently high hydrogencontent for being directed to a methanation unit and subsequenthydrogenation reactor. The membranes may also comprise polyimide hollowfiber membranes.

Beneficial Effects

1. The components that may damage the membrane, such as methyl iodide,acetic acid, water, in the high-pressure tail gas of carbonylation toacetic acid preferably are substantially removed through the gas washingcolumn. In a preferred embodiment, the high-pressure acetic acid tailgas at a stable flow rate is charged into an adsorption column having atray structure and equipped with a pre-treatment system, and contactswith top-down desalted water reversely in the column. In this manner,harmful components in the tail gas, such as acetic acid and water, maybe removed by controlling the circulation of water supply such that theacetic acid content in the tail gas is equal to or less than 100 ppm,e.g., equal to or less than 50 ppm.

2. Droplets entrained in the tail gas may be removed by the misteliminator, to reduce or eliminate the influence of such droplets on theeffect of membrane separation and damage on the membrane components.

3. Impurity particulates in the tail gas preferably are removed by thecombined filter to reduce and minimize the potential for damage to themembrane components.

4. The high-pressure mixed acetic acid tail gas is preferably at apressure of 2.6-2.8 MPa and a flow rate of 2000 m³/h-6000 m³/h. The flowrate of the recovered carbon monoxide product gas and/or recoveredhydrogen stream preferably is controlled at 1000 m³/h-3000 m³/h to avoidthe influence of N₂ accumulation on the long-term stable running of themembrane system.

5. In this process, by using a vapor heater, the temperature of themixed gas is preferably increased so as to avoid the water dew point toenter into the membrane system for separation. After being charged intothe shell of the membrane separator, the mixed tail gas flows alongoutside of the fiber, and hydrogen gas selectively and preferentiallypermeates through the wall of the fiber membrane and is enriched at alow-pressure side in the pipe. The resulting hydrogen stream is thenintroduced out of the membrane separation system as a feed to the aceticacid hydrogenation step, described below. In a preferred embodiment, thehydrogen stream is directed to a methanation unit for reducing theamount of any residual carbon monoxide that is contained therein.Meanwhile, carbon monoxide and nitrogen with low permeation rate arereserved at a non-permeated gas side and have a pressure similar to thatof the raw gas and are introduced out of the system as carbon monoxideproduct gas for compression and optionally recycle to the carbonylationstep. According to the present invention, the recovery rate iscontrolled by adjusting the flow rates of the raw gas, the hydrogen-richgas stream and the carbon monoxide product gas, and the pressure of thecarbon monoxide product gas is increased through a gas compressor to bebalanced with the system pressure and integrated into the system.

6. The process for recovery and reuse of acetic acid high-pressure tailgas with membranes according to the present invention has the additionaladvantages that there is no phase transition, the energy consumption islow and the unit scale can be adjusted according to the requirement ofthe gas amount being treated, compared with pressure swing adsorptionseparation technique. Furthermore, the process has the advantage ofbeing a simple unit, with convenient operation, and high runningreliability.

A non-limiting exemplary separation system according to one embodimentof the invention is provided below with reference to FIGS. 4 and 5.

1. Gas washing column T1

The flow rate of the acetic acid mixed high-pressure tail gas ispreferably 2000 m³/h-6000 m³/h, the working pressure of the gas washingcolumn is preferably 2.6-2.8 MPa, the temperature is preferably 40-60°C. The column has 22 stainless steel floating valve trays therein, andthe mass content of acetic acid in the absorption solution after theacetic acid high-pressure tail gas passes through the gas washing columnis preferably about 30%.

Desalted water is pumped to the top of the gas washing column by acirculation pump of the column and contacts the acetic acidhigh-pressure tail gas reversely. The circulation of the desalted wateris preferably 0.5-1.0 m³/h and may be adjusted according to the aceticacid content in the tail gas, such that the acetic acid content in theacetic acid high-pressure tail gas at the outlet of the gas washingcolumn is equal to or less than 100 wppm, e.g., equal to or less than 50wppm.

The flow rate of the water supply of the gas washing column iscontrolled to be stable, the liquid level at the bottom is stabilized,and the waste acid solution is recovered for reuse.

2. Mist eliminator X1: The mist eliminator can remove droplets entrainedin the acetic acid high-pressure tail gas effectively. The liquid levelof the mist eliminator is preferably controlled at 150-200 mm, and theliquid is discharged regularly. The separator has an interlockprotection device for liquid level to prevent severe gas-liquidentrainment from causing adverse effect on the membrane life.

3. Combined filter X1B: The combined filter can remove particulatesentrained in the acetic acid high-pressure tail gas to prevent them fromblocking the membranes and influencing the membrane separation effect.The filter precision is preferably about 0.01 μm, and the stability ofthe filter is ensured by monitoring the pressure difference.

4. Heater E1: After the above processing, the acetic acid high-pressuretail gas is not a saturated gas, and water will impact the use of themembrane. At 0.6 MPa low-pressure vapor, the tail gas is preferablyheated to 40-60° C. such that the tail gas avoids the dew point. Thecontrol, indication, alarm, and interlock of temperature may be realizedby using a vapor flow rate regulating valve and a temperature transducerso as to maintain the gas temperature at constant value. The coolingwater of the entire unit may be discharged to waste water treatment.

5. Prism membrane separator M1

The separation membrane used in the unit may comprise any suitablemembranes for separating carbon monoxide from hydrogen. In one aspect,the membranes comprise hollow fiber membranes, e.g., Prism membranes,and the membrane material comprises polyimide. The separator includes atotal of at least 32 membrane components in two sets. Additionalmembranes and stages may be employed to obtain the desiredhydrogen/carbon monoxide separation.

The pressure increasing rate of the membrane separator preferably isequal to or less than 0.3 MPa/min. When the pressure of the membraneseparator is increased to be balanced with the pressure of the previoussystem, it is required to open the raw gas inlet valve, product carbonmonoxide outlet valve, and hydrogen-rich gas outlet valve of each set ofmembrane separator to be used.

The pressure, flow rate, and carbon monoxide content of the product gasare controlled and adjusted through a regulating valve, to ensure thepressure at the inlet of the tail gas compressor to be stable.

As discussed, each of the different embodiments of the invention mayemploy one or more membrane separation units to separate a hydrogenstream and a carbon monoxide stream from a process stream. In the thirdembodiment, the one or more separation units separate a hydrogen streamand a carbon monoxide stream from a syngas stream. The syngas streambefore separation optionally has a hydrogen to carbon monoxide molarratio less than 3:1, less than 2:1, less than 1:1 or less than 0.5:1.The processes of the invention, however, provide the ability to form aseparated hydrogen stream (before or after optional methanation) havinga ratio preferably greater than 2:1, greater than 5:1, or greater than10:1, and optionally a carbon monoxide stream having a ratio less than1:1, less than 0.5:1, less than 0.25:1 or less than 0.1:1.

6. Compressor C1

The flow rate of the product gas is preferably 1000 m³/h-3000 m³/h, andthe pressure of the membrane carbon monoxide product gas is increased bythe tail gas compressor from 2.1-2.3 MPa to the use pressure of the nextstep, and it is combined into the acetic acid system for recovery andreuse.

Control of the recovery amount and recovery rate of the acetic acid tailgas.

Recovery amount: The membrane recovery unit can realize the recovery andreuse of carbon monoxide, but nitrogen gas cannot be separatedeffectively, thus causing accumulation of nitrogen gas in the system.According to the present invention, continuous recovery of the tail gasis realized by adjusting the torch venting amount and the recoveryamount of the tail gas. Adjustment and balance of the recovery amount ofthe high-pressure tail gas of the unit are realized through thecalculation of the nitrogen material balance in the acetic acid carbonmonoxide pipe of the current unit, thus keeping the recovery amount ofnitrogen and the venting amount of the hydrogen-rich gas to beconsistent.

Recovery rate: The Prism membrane system can ensure long-term carbonmonoxide recovery from acetic acid high-pressure tail gas at the largestamount, and the carbon monoxide content to be equal to or greater than91%. By adjusting gas amount fed into the membrane and the product gasamount, the acetic acid high-pressure tail gas is recovered and reusedat the maximum extent, while ensuring the carbon monoxide proportion inthe acetic acid pipe.

With the pre-treatment system consisting of the gas washing column, themist eliminator, and the combined filter, “harmful” components in thetail gas are removed, to prevent the influence on the stable running ofthe membrane. In practice, multiple sets of membranes are adopted torealize effective separation of the tail gas, and the processed gasamount, the carbon monoxide product gas composition, and the hydrogenstream meet the working requirements. The material balance may becalculated and the gas amount of the product gas fed into acetic acidmay be adjusted over time to prevent N₂ accumulation.

Methanation of Hydrogen Stream

As indicated above, the hydrogen stream from the membrane separator maycomprise substantial amounts of carbon monoxide in addition to hydrogen.Depending on the catalyst employed in the subsequent hydrogenationprocess, carbon monoxide may not be well tolerated in the hydrogenationstep. As a result, in the first embodiment of the invention, it may bedesired to reduce the amount of carbon monoxide contained in thehydrogen stream prior to sending the hydrogen stream to thehydrogenation process. One technique for reducing the carbon monoxidecontent of the hydrogen stream includes methanation, and in a preferredembodiment, the invention includes a step of methanating the hydrogenstream prior to directing it to the hydrogenation process.

In the second embodiment of the invention, a crude ethanol product isdirected from the hydrogenation reactor to a separator unit, e.g., aflash vessel, which forms a vapor stream and a liquid ethanol productstream. The vapor stream is separated into a carbon monoxide stream anda hydrogen stream, which may comprise residual carbon monoxide. Sincecarbon monoxide may not be well tolerated in the hydrogenation process,the carbon monoxide content of the hydrogen stream also may be furtherreduced through methanation.

In the third embodiment of the invention, a syngas stream comprisinghydrogen and carbon monoxide is separated in a separation unit,preferably comprising one or more membrane separation units, to form acarbon monoxide stream and a hydrogen stream, which may compriseresidual carbon monoxide. As with the above embodiments, since carbonmonoxide may not be well tolerated in the hydrogenation process, thecarbon monoxide content of the hydrogen stream also may be furtherreduced through methanation.

Processes for methanating carbon monoxide-containing streams aredescribed, for example, in A. Rehmat et al., “Selective Methanation ofCarbon Monoxide,” Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4,pp. 512-515 (1970), the entirety of which is incorporated herein byreference.

During the methanation process, each mole of carbon monoxide reacts withthree moles of hydrogen to form methane, which is unreactive undertypical hydrogenation conditions, and water. Thus, hydrogen should bepresent in the hydrogen stream in at least a 3:1 molar excess relativeto the amount of carbon monoxide contained in the hydrogen stream andshould have sufficient excess hydrogen in order to have residualhydrogen remaining in the methanated hydrogen stream to serve as ahydrogen source for the hydrogenation reaction. In some exemplaryembodiments, the molar ratio of hydrogen to carbon monoxide in thehydrogen stream before methanation is at least 3.5:1, at least 4:1 or atleast 5:1.

The methanation step is preferably conducted in a methanation reactorunder methanation conditions. The methanation conditions may varywidely, but in one embodiment, the methanation step is conducted at atemperature from 125° C. to 300° C., which provides a negligible amountof reverse shift reaction while providing vigorous carbon monoxidemethanation. Because the methanation reaction is exothermic, thetemperature will increase over the course of the reaction. Preferably,the reaction is maintained at a methanation temperature below 300° C.,preferably below 250° C., or a temperature between 200° C. and 250° C.At greater temperatures, more carbon monoxide may be undesirablyproduced.

The methanation step preferably is conducted in the presence of amethanation catalyst. Exemplary methanation catalysts may include one ormore metals selected from the group consisting of ruthenium, nickel andaluminum. For example, the catalyst may comprise ruthenium (e.g.,ruthenium on a-alumina), IGT Raney nickel (e.g., 30-35% nickel, 5%aluminum, and 60% Al₂O₃-3H₂O), Harshaw nickel (e.g., 58% Ni onkieselguhr), CCI nickel (e.g., 47% nickel oxide on alumina), or GirdlerG-65 (nickel on a-alumina).

Although the composition of the hydrogen stream after methanation mayvary depending on the composition of the initial hydrogen stream and themethanation conditions, the methanated hydrogen stream for the firstembodiment preferably comprises at least 85 vol. % hydrogen, e.g., atleast 90 vol. % hydrogen or at least 95 vol. % hydrogen. The methanatedhydrogen stream for the first embodiment preferably comprises less than2 vol. % carbon monoxide, e.g., less than 1 vol. % carbon monoxide orless than 0.5 vol. % carbon monoxide. The methanated hydrogen stream forthe second embodiment will generally comprise a greater amount ofhydrogen, and preferably comprises at least 90 vol. % hydrogen, e.g., atleast 95 vol. % hydrogen or at least 99 vol. % hydrogen. The methanatedhydrogen stream for the second embodiment preferably comprises less than2 vol. % carbon monoxide, e.g., less than 1 vol. % carbon monoxide orless than 0.5 vol. % carbon monoxide.

Hydrogenation of Acetic Acid

As discussed above, in the first embodiment, hydrogen derived from thecarbonylation process tail gas preferably is used in the hydrogenationof acetic acid to form ethanol. In the second embodiment, hydrogenderived from the separator vapor stream preferably is recycled to thehydrogenation reactor. The acetic acid fed to the hydrogenation reactionmay also comprise other carboxylic acids and anhydrides, as well asacetaldehyde and acetone. Preferably, a suitable acetic acid feed streamcomprises one or more of the compounds selected from the groupconsisting of acetic acid, acetic anhydride, acetaldehyde, ethylacetate, and mixtures thereof. These other compounds may also behydrogenated in the processes of the present invention. In someembodiments, the presence of carboxylic acids, such as propanoic acid orits anhydride, may be beneficial in producing propanol. Water may alsobe present in the acetic acid feed.

Alternatively, acetic acid in vapor form may be taken directly as crudeproduct from the flash vessel of a methanol carbonylation unit of theclass described in U.S. Pat. No. 6,657,078, the entirety of which isincorporated herein by reference. The crude vapor product, for example,may be fed directly to the ethanol synthesis reaction zones of thepresent invention without the need for condensing the acetic acid andlight ends or removing water, saving overall processing costs.

The acetic acid may be vaporized at the reaction temperature, followingwhich the vaporized acetic acid may be fed along with hydrogen in anundiluted state or diluted with a relatively inert carrier gas, such asnitrogen, argon, helium, carbon dioxide and the like. For reactions runin the vapor phase, the temperature should be controlled in the systemsuch that it does not fall below the dew point of acetic acid. In oneembodiment, the acetic acid may be vaporized at the boiling point ofacetic acid at the particular pressure, and then the vaporized aceticacid may be further heated to the reactor inlet temperature. In anotherembodiment, the acetic acid is mixed with other gases before vaporizing,followed by heating the mixed vapors up to the reactor inlettemperature. Preferably, the acetic acid is transferred to the vaporstate by passing hydrogen and/or recycle gas through the acetic acid ata temperature at or below 125° C., followed by heating of the combinedgaseous stream to the reactor inlet temperature.

Some embodiments of the process of hydrogenating acetic acid to formethanol may include a variety of configurations using a fixed bedreactor or a fluidized bed reactor. In many embodiments of the presentinvention, an “adiabatic” reactor can be used; that is, there is littleor no need for internal plumbing through the reaction zone to add orremove heat. In other embodiments, a radial flow reactor or reactors maybe employed, or a series of reactors may be employed with or withoutheat exchange, quenching, or introduction of additional feed material.Alternatively, a shell and tube reactor provided with a heat transfermedium may be used. In many cases, the reaction zone may be housed in asingle vessel or in a series of vessels with heat exchangerstherebetween.

In preferred embodiments, the catalyst is employed in a fixed bedreactor, e.g., in the shape of a pipe or tube, where the reactants,typically in the vapor form, are passed over or through the catalyst.Other reactors, such as fluid or ebullient bed reactors, can beemployed. In some instances, the hydrogenation catalysts may be used inconjunction with an inert material to regulate the pressure drop of thereactant stream through the catalyst bed and the contact time of thereactant compounds with the catalyst particles.

The hydrogenation reaction may be carried out in either the liquid phaseor vapor phase. Preferably, the reaction is carried out in the vaporphase under the following conditions. The reaction temperature may rangefrom 125° C. to 350° C., e.g., from 200° C. to 325° C., from 225° C. to300° C., or from 250° C. to 300° C. The pressure may range from 10 kPato 3000 kPa, e.g., from 50 kPa to 2300 kPa, or from 100 kPa to 1500 kPa.The reactants may be fed to the reactor at a gas hourly space velocity(GHSV) of greater than 500 hr⁻¹, e.g., greater than 1000 hr⁻¹, greaterthan 2500 hr⁻¹ or even greater than 5000 hr⁻¹. In terms of ranges theGHSV may range from 50 hr⁻¹ to 50,000 hr⁻¹, e.g., from 500 hr⁻¹ to30,000 hr⁻¹, from 1000 hr⁻¹ to 10,000 hr⁻¹, or from 1000 hr⁻¹ to 6500hr⁻¹.

The hydrogenation optionally is carried out at a pressure justsufficient to overcome the pressure drop across the catalytic bed at theGHSV selected, although there is no bar to the use of higher pressures,it being understood that considerable pressure drop through the reactorbed may be experienced at high space velocities, e.g., 5000 hr⁻¹ or6,500 hr⁻¹.

Although the reaction consumes two moles of hydrogen per mole of aceticacid to produce one mole of ethanol, the actual molar ratio of hydrogento acetic acid in the feed stream may vary from about 100:1 to 1:100,e.g., from 50:1 to 1:50, from 20:1 to 1:2, or from 12:1 to 1:1. Mostpreferably, the molar ratio of hydrogen to acetic acid is greater than2:1, e.g., greater than 4:1 or greater than 8:1.

Contact or residence time can also vary widely, depending upon suchvariables as amount of acetic acid, catalyst, reactor, temperature, andpressure. Typical contact times range from a fraction of a second tomore than several hours when a catalyst system other than a fixed bed isused, with preferred contact times, at least for vapor phase reactions,of from 0.1 to 100 seconds, e.g., from 0.3 to 80 seconds or from 0.4 to30 seconds.

The hydrogenation of acetic acid to form ethanol is preferably conductedin the presence of a hydrogenation catalyst. Suitable hydrogenationcatalysts include catalysts comprising a first metal and optionally oneor more of a second metal, a third metal or any number of additionalmetals, optionally on a catalyst support. The first and optional secondand third metals may be selected from Group IB, JIB, IIIB, IVB, VB, VIB,VIIB, VIII transition metals, a lanthanide metal, an actinide metal or ametal selected from any of Groups IIIA, IVA, VA, and VIA. In oneembodiment, the catalyst comprises platinum and/or palladium. Preferredmetal combinations for some exemplary catalyst compositions includeplatinum/tin, platinum/ruthenium, platinum/rhenium, palladium/ruthenium,palladium/rhenium, cobalt/palladium, cobalt/platinum, cobalt/chromium,cobalt/ruthenium, cobalt/tin, silver/palladium, copper/palladium,copper/zinc, nickel/palladium, gold/palladium, ruthenium/rhenium, andruthenium/iron. Exemplary catalysts are further described in U.S. Pat.No. 7,608,744 and U.S. Pub. No. 2010/0029995, the entireties of whichare incorporated herein by reference. In another embodiment, thecatalyst comprises a Co/Mo/S catalyst of the type described in U.S. Pub.No. 2009/0069609, the entirety of which is incorporated herein byreference.

In one embodiment, the catalyst comprises a first metal selected fromthe group consisting of copper, iron, cobalt, nickel, ruthenium,rhodium, palladium, osmium, iridium, platinum, titanium, zinc, chromium,rhenium, molybdenum, and tungsten. Preferably, the first metal isselected from the group consisting of platinum, palladium, cobalt,nickel, and ruthenium. More preferably, the first metal is selected fromplatinum and palladium. In embodiments of the invention where the firstmetal comprises platinum, it is preferred that the catalyst comprisesplatinum in an amount less than 5 wt. %, e.g., less than 3 wt. % or lessthan 1 wt. %, due to the high commercial demand for platinum.

As indicated above, in some embodiments, the catalyst further comprisesa second metal, which typically would function as a promoter. Ifpresent, the second metal preferably is selected from the groupconsisting of copper, molybdenum, tin, chromium, iron, cobalt, vanadium,tungsten, palladium, platinum, lanthanum, cerium, manganese, ruthenium,rhenium, gold, and nickel. More preferably, the second metal is selectedfrom the group consisting of copper, tin, cobalt, rhenium, and nickel.More preferably, the second metal is selected from tin and rhenium.

In certain embodiments where the catalyst includes two or more metals,e.g., a first metal and a second metal, the first metal is present inthe catalyst in an amount from 0.1 to 10 wt. %, e.g., from 0.1 to 5 wt.%, or from 0.1 to 3 wt. %. The second metal preferably is present in anamount from 0.1 to 20 wt. %, e.g., from 0.1 to 10 wt. %, or from 0.1 to5 wt. %. For catalysts comprising two or more metals, the two or moremetals may be alloyed with one another or may comprise a non-alloyedmetal solution or mixture.

The preferred metal ratios may vary depending on the metals used in thecatalyst. In some exemplary embodiments, the mole ratio of the firstmetal to the second metal is from 10:1 to 1:10, e.g., from 4:1 to 1:4,from 2:1 to 1:2, from 1.5:1 to 1:1.5 or from 1.1:1 to 1:1.1.

The catalyst may also comprise a third metal selected from any of themetals listed above in connection with the first or second metal, solong as the third metal is different from the first and second metals.In preferred aspects, the third metal is selected from the groupconsisting of cobalt, palladium, ruthenium, copper, zinc, platinum, tin,and rhenium. More preferably, the third metal is selected from cobalt,palladium, and ruthenium. When present, the total weight of the thirdmetal preferably is from 0.05 to 4 wt. %, e.g., from 0.1 to 3 wt. %, orfrom 0.1 to 2 wt. %.

In addition to one or more metals, in some embodiments of the presentinvention the catalysts further comprise a support or a modifiedsupport. As used herein, the term “modified support” refers to a supportthat includes a support material and a support modifier, which adjuststhe acidity of the support material.

The total weight of the support or modified support, based on the totalweight of the catalyst, preferably is from 75 to 99.9 wt. %, e.g., from78 to 97 wt. %, or from 80 to 95 wt. %. In preferred embodiments thatutilize a modified support, the support modifier is present in an amountfrom 0.1 to 50 wt. %, e.g., from 0.2 to 25 wt. %, from 0.5 to 15 wt. %,or from 1 to 8 wt. %, based on the total weight of the catalyst. Themetals of the catalysts may be dispersed throughout the support, layeredthroughout the support, coated on the outer surface of the support(i.e., egg shell), or decorated on the surface of the support.

As will be appreciated by those of ordinary skill in the art, supportmaterials are selected such that the catalyst system is suitably active,selective and robust under the process conditions employed for theformation of ethanol.

Suitable support materials may include, for example, stable metaloxide-based supports or ceramic-based supports. Preferred supportsinclude silicaceous supports, such as silica, silica/alumina, a GroupIIA silicate such as calcium metasilicate, pyrogenic silica, high puritysilica, and mixtures thereof. Other supports may include, but are notlimited to, iron oxide, alumina, titania, zirconia, magnesium oxide,carbon, graphite, high surface area graphitized carbon, activatedcarbons, and mixtures thereof.

As indicated, the catalyst support may be modified with a supportmodifier. In some embodiments, the support modifier may be an acidicmodifier that increases the acidity of the catalyst. Suitable acidicsupport modifiers may be selected from the group consisting of: oxidesof Group IVB metals, oxides of Group VB metals, oxides of Group VIBmetals, oxides of Group VIIB metals, oxides of Group VIIIB metals,aluminum oxides, and mixtures thereof. Acidic support modifiers includethose selected from the group consisting of TiO₂, ZrO₂, Nb₂O₅, Ta₂O₅,Al₂O₃, B₂O₃, P₂O₅, and Sb₂O₃. Preferred acidic support modifiers includethose selected from the group consisting of TiO₂, ZrO₂, Nb₂O₅, Ta₂O₅,and Al₂O₃. The acidic modifier may also include WO₃, MoO₃, Fe₂O₃, Cr₂O₃,V₂O₅, MnO₂, CuO, Co₂O₃, and Bi₂O₃.

In another embodiment, the support modifier may be a basic modifier thathas a low volatility or no volatility. Such basic modifiers, forexample, may be selected from the group consisting of: (i) alkalineearth oxides, (ii) alkali metal oxides, (iii) alkaline earth metalmetasilicates, (iv) alkali metal metasilicates, (v) Group JIB metaloxides, (vi) Group IIB metal metasilicates, (vii) Group IIIB metaloxides, (viii) Group IIIB metal metasilicates, and mixtures thereof. Inaddition to oxides and metasilicates, other types of modifiers includingnitrates, nitrites, acetates, and lactates may be used. Preferably, thesupport modifier is selected from the group consisting of oxides andmetasilicates of any of sodium, potassium, magnesium, calcium, scandium,yttrium, and zinc, as well as mixtures of any of the foregoing. Morepreferably, the basic support modifier is a calcium silicate, and evenmore preferably calcium metasilicate (CaSiO₃). If the basic supportmodifier comprises calcium metasilicate, it is preferred that at least aportion of the calcium metasilicate is in crystalline form.

A preferred silica support material is SS61138 High Surface Area (HSA)Silica Catalyst Carrier from Saint Gobain NorPro. The Saint-GobainNorPro SS61138 silica exhibits the following properties: containsapproximately 95 wt. % high surface area silica; surface area of about250 m²/g; median pore diameter of about 12 nm; average pore volume ofabout 1.0 cm³/g as measured by mercury intrusion porosimetry and apacking density of about 0.352 g/cm³ (22 lb/ft³).

A preferred silica/alumina support material is KA-160 silica spheresfrom Sud Chemie having a nominal diameter of about 5 mm, a density ofabout 0.562 g/ml, an absorptivity of about 0.583 g H₂O/g support, asurface area of about 160 to 175 m²/g, and a pore volume of about 0.68ml/g.

The catalyst compositions suitable for use with the present inventionpreferably are formed through metal impregnation of the modifiedsupport, although other processes such as chemical vapor deposition mayalso be employed. Such impregnation techniques are described in U.S.Pat. Nos. 7,608,744 and 7,863,489 and U.S. Pub. No. 2010/0197485referred to above, the entireties of which are incorporated herein byreference.

In particular, the hydrogenation of acetic acid may achieve favorableconversion of acetic acid and favorable selectivity and productivity toethanol. For purposes of the present invention, the term “conversion”refers to the amount of acetic acid in the feed that is converted to acompound other than acetic acid. Conversion is expressed as a molepercentage based on acetic acid in the feed. The conversion may be atleast 10%, e.g., at least 20%, at least 40%, at least 50%, at least 60%,at least 70% or at least 80%. Although catalysts that have highconversions are desirable, such as at least 80% or at least 90%, in someembodiments a low conversion may be acceptable at high selectivity forethanol. It is, of course, well understood that in many cases, it ispossible to compensate for conversion by appropriate recycle streams oruse of larger reactors, but it is more difficult to compensate for poorselectivity.

Selectivity is expressed as a mole percent based on converted aceticacid. It should be understood that each compound converted from aceticacid has an independent selectivity and that selectivity is independentfrom conversion. For example, if 60 mole % of the converted acetic acidis converted to ethanol, we refer to the ethanol selectivity as 60%.Preferably, the catalyst selectivity to ethoxylates is at least 60%,e.g., at least 70%, or at least 80%. As used herein, the term“ethoxylates” refers specifically to the compounds ethanol,acetaldehyde, and ethyl acetate. Preferably, the selectivity to ethanolis at least 80%, e.g., at least 85% or at least 88%. Preferredembodiments of the hydrogenation process also have low selectivity toundesirable products, such as methane, ethane, and carbon dioxide. Theselectivity to these undesirable products preferably is less than 4%,e.g., less than 2% or less than 1%. More preferably, these undesirableproducts are present in undetectable amounts. Formation of alkanes maybe low, and ideally less than 2%, less than 1%, or less than 0.5% of theacetic acid passed over the catalyst is converted to alkanes, which havelittle value other than as fuel.

The term “productivity,” as used herein, refers to the grams of aspecified product, e.g., ethanol, formed during the hydrogenation basedon the kilograms of catalyst used per hour. A productivity of at least100 grams of ethanol per kilogram of catalyst per hour, e.g., at least400 grams of ethanol per kilogram of catalyst per hour or at least 600grams of ethanol per kilogram of catalyst per hour, is preferred. Interms of ranges, the productivity preferably is from 100 to 3,000 gramsof ethanol per kilogram of catalyst per hour, e.g., from 400 to 2,500grams of ethanol per kilogram of catalyst per hour or from 600 to 2,000grams of ethanol per kilogram of catalyst per hour.

Operating under the conditions of the present invention may result inethanol production on the order of at least 0.1 tons of ethanol perhour, e.g., at least 1 ton of ethanol per hour, at least 5 tons ofethanol per hour, or at least 10 tons of ethanol per hour. Larger scaleindustrial production of ethanol, depending on the scale, generallyshould be at least 1 ton of ethanol per hour, e.g., at least 15 tons ofethanol per hour or at least 30 tons of ethanol per hour. In terms ofranges, for large scale industrial production of ethanol, the process ofthe present invention may produce from 0.1 to 160 tons of ethanol perhour, e.g., from 15 to 160 tons of ethanol per hour or from 30 to 80tons of ethanol per hour. Ethanol production from fermentation, due theeconomies of scale, typically does not permit the single facilityethanol production that may be achievable by employing embodiments ofthe present invention.

In various embodiments of the present invention, the crude ethanolproduct produced by the hydrogenation process, before any subsequentprocessing, such as purification and separation, will typically compriseunreacted acetic acid, ethanol and water. As used herein, the term“crude ethanol product” refers to any composition comprising from 5 to70 wt. % ethanol and from 5 to 40 wt. % water. Exemplary compositionalranges for the crude ethanol product are provided in Table 1. The“others” identified in Table 1 may include, for example, esters, ethers,aldehydes, ketones, alkanes, and carbon dioxide.

TABLE 1 CRUDE ETHANOL PRODUCT COMPOSITIONS Conc. Conc. Conc. Conc.Component (wt. %) (wt. %) (wt. %) (wt. %) Ethanol 5 to 70 15 to 70  15to 50 25 to 50 Acetic Acid 0 to 90 0 to 50 15 to 70 20 to 70 Water 5 to40 5 to 30 10 to 30 10 to 26 Ethyl Acetate 0 to 30 0 to 20  1 to 12  3to 10 Acetaldehyde 0 to 10 0 to 3  0.1 to 3   0.2 to 2   Others 0.1 to10   0.1 to 6   0.1 to 4   —

In one embodiment, the crude ethanol product may comprise acetic acid inan amount less than 20 wt. %, e.g., of less than 15 wt. %, less than 10wt. % or less than 5 wt. %. In embodiments having lower amounts ofacetic acid, the conversion of acetic acid is preferably greater than75%, e.g., greater than 85% or greater than 90%. In addition, theselectivity to ethanol may also be preferably high, and is greater than75%, e.g., greater than 85% or greater than 90%.

Several different separation schemes may be employed in purifying thecrude ethanol product of the present invention. See, for example, USPublished Application US20110190547, the entirety of which isincorporated herein by reference, and U.S. patent application Ser. Nos.12/852,305, 13/094,691, 13/094,537, 13/094,588, and 13/094,657, theentireties of which are incorporated herein by reference. As discussedabove, the separation scheme preferably includes a separator unit, e.g.,a flash vessel, for separating a vapor stream from a liquid crudeethanol product. The vapor stream preferably comprises hydrogen andcarbon monoxide, which according to the second embodiment of theinvention may be separated in one or more membrane separators to form ahydrogen stream and a carbon monoxide stream. The carbon monoxide streampreferably is recycled to the carbonylation reactor and the hydrogenstream preferably is recycled to the hydrogenation reactor, optionallyafter methanation thereof to remove residual carbon monoxide.

While the invention has been described in detail, modifications withinthe spirit and scope of the invention will be readily apparent to thoseof skill in the art. In view of the foregoing discussion, relevantknowledge in the art and references discussed above in connection withthe Background and Detailed Description, the disclosures of which areall incorporated herein by reference. In addition, it should beunderstood that aspects of the invention and portions of variousembodiments and various features recited below and/or in the appendedclaims may be combined or interchanged either in whole or in part. Inthe foregoing descriptions of the various embodiments, those embodimentswhich refer to another embodiment may be appropriately combined withother embodiments as will be appreciated by one of skill in the art.Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tolimit the invention.

We claim:
 1. A process for forming an alcohol, comprising: (a)separating a syngas stream comprising carbon monoxide and hydrogen intoa hydrogen stream comprising hydrogen and residual carbon monoxide, anda carbon monoxide stream comprising carbon monoxide; (b) methanating atleast a portion of the hydrogen stream to convert at least a portion ofthe residual carbon monoxide to methane and water and forming amethanated hydrogen stream; and (c) hydrogenating an alkanoic acid withhydrogen from the methanated hydrogen stream in the presence of acatalyst under conditions effective to form the alcohol.
 2. The processof claim 1, wherein the alkanoic acid is acetic acid and the alcohol isethanol.
 3. The process of claim 1, further comprising carbonylating analcohol having n carbon atoms with carbon monoxide from the carbonmonoxide stream to form the alkanoic acid, wherein the alkanoic acid andthe alcohol formed in the hydrogenating step comprise n+1 carbon atoms.4. The process of claim 1, wherein the separating comprises membraneseparating.
 5. The process of claim 1, wherein the methanated streamcomprises at least 85 vol. % hydrogen.
 6. The process of claim 1,wherein the methanated stream comprises less than 2 vol. % carbonmonoxide.
 7. The process of claim 1, wherein the methanol is derivedfrom a carbon source selected from the group consisting of natural gas,oil, petroleum, coal, biomass, and combinations thereof.
 8. The processof claim 1, wherein the hydrogenation catalyst comprises platinum orpalladium.
 9. A process for forming an alcohol from an alkanoic acid,comprising separating and methanating syngas to form a methanatedhydrogen stream and hydrogenating the alkanoic acid to form the alcohol,wherein at least some hydrogen for the hydrogenating step is derivedfrom the methanated hydrogen stream.
 10. The process of claim 9, whereinthe alkanoic acid is acetic acid and the alcohol is ethanol.
 11. Theprocess of claim 9, wherein the separating comprises separating thesyngas into a hydrogen stream and a carbon monoxide stream, and whereinthe hydrogen stream is methanated.
 12. The process of claim 11, furthercomprising carbonylating an alcohol having n carbon atoms with carbonmonoxide from the carbon monoxide stream to form the alkanoic acid,wherein the alkanoic acid and the alcohol formed in the hydrogenatingstep comprise n+1 carbon atoms.
 13. The process of claim 11, wherein theseparating comprises membrane separating.
 14. The process of claim 9,wherein the methanated stream comprises at least 85 vol. % hydrogen. 15.The process of claim 9, wherein the methanated stream comprises lessthan 2 vol. % carbon monoxide.
 16. The process of claim 9, wherein themethanol is derived from a carbon source selected from the groupconsisting of natural gas, oil, petroleum, coal, biomass, andcombinations thereof.
 17. The process of claim 9, wherein thehydrogenating occurs in the presence of a catalyst comprising platinumor palladium.